Recently, in computers and storage systems, semiconductor storage media enabling I/O accesses at speeds higher than HDDs (Hard Disk Drives) are used. For example, SSDs (Solid State Drives) utilizing NAND type flash memories are a typical example of such media. The flash memory stores data by applying high voltage to floating gates enclosed via insulator films, to move electrons in and out. Insulator films are worn by the forced passing of electrons, so that there is a limitation in the rewritable frequency of data. Along with the refinement of processes, the rewritable frequency is being reduced. Therefore, in storage systems using SSDs and other flash memories, it is necessary to perform operation considering the rewrite life in addition to the management of capacity.
Therefore, in cases where the rewrite life is ended prior to the aging life of a product (which is five years, in the case of enterprise products), the user had to cope with the problem by either one of the following two methods.
One method is to wait for failure replacement. Generally, redundancy is realized in a storage subsystem via a RAID (Redundant Array of Independent/Inexpensive Disks) technique and the like, so that the failure will not directly lead to data loss. Therefore, the drive having reached its life is blocked as failure and replaced with a new drive, to thereby elongate life of the subsystem. When a drive is blocked, the RAID uses a new drive to perform a rebuilding process, so the subsystem must allow deterioration of processing performance and deterioration of availability until the process is completed. Patent Literature 1 teaches an art of performing maintenance in a preventive manner by copying data to a spare drive before failure. However, the concept of the method for coping with the problem of life is the same as failure replacement.
The other method is a method for further adding a flash memory drive or a RAID configuration group composed of multiple flash memory drives, realizing elongation of life by performing load distribution of the amount of rewrites. Generally, in a flash memory drive, there is a technique of performing wear leveling using flash memory chips installed within the drive without deviation, wherein a technique for leveling the number of rewrites (number of erases) among multiple flash memory drives (FM packages) is taught, for example, in Patent Literature 2.